A U.N. report released Tuesday documents cases of children recruited and used as soldiers by eight national armies and 51 armed groups in the past year.

Some of the most serious situations are reported in Syria, Iraq, Nigeria and South Sudan.

According to Leila Zerrougui, U.N. envoy on children and armed conflict, grave rights violations of children, including death, maiming, sexual violence and attacks on schools and hospitals, are highlighted.

In Syria, where a civil war is raging into its fourth year, the U.N. estimates more than 10,000 children have been killed.

"The conflict and violence in Syria reached unprecedented levels during 2013 — no sign of improvement since the beginning of the year and no political solution in sight," Zerrougui said.

In her report, Zerrougi says armed groups, including several associated with the opposition Free Syrian Army, as well as Islamist groups ISIL and Jhabat al-Nusra, are recruiting and using children for logistics, handling ammunition, manning checkpoints and as combatants. Armed groups also continue to kill and injure children, rape girls and target schools.

In neighboring Iraq, where ISIL fighters recently have made bold military gains, children are facing extremely violent and dangerous conditions. The report says nearly 8,000 civilians were killed last year, and at least 250 of them were children.

Terror groups in Africa also are responsible for attacks on children. Most notably in Nigeria, where Boko Haram, which has kidnapped scores of school girls in the country's north, has repeatedly targeted schools. According to the U.N. report, the group also has children as young as 12 in its ranks.

In South Sudan, Zerrougi said gains in reintegrating child soldiers back into society have been reversed by the political conflict that erupted in mid-December.

"I saw [for] myself children with guns; with the government, but also with armed groups — with David Yau Yau, with the SSLA and with the opposition under Riek Machar," she said.

On a positive note, Chad, which has previously been named for its use of child soldiers, was de-listed by the U.N. this year. The government signed onto an action plan with the U.N. in 2011 to end the practice.

Chad is an important troop contributor to U.N. peacekeeping operations, and its record as a recruiter of child soldiers threatened to jeopardize its participation in U.N. missions.